3.1 Cumulative Costs
At each instant of time, the cumulative costs represent the
sum of all costs of a given type from the beginning of service
up to that certain instant of time. In general, these costs
gradually increase with the operating time. In experiments (in
service), the cumulative costs are usually the easiest to
identify, as it is the most often evaluated economic value.
Dependency of cumulative costs upon the operating time
usually does not have a continuous character, and therefore
various mathematical models often substitute for this
dependency.